Radiation therapy (also referred to as radiotherapy) may be used in the treatment of cancer or other pathologies. Radiotherapy involves delivering a prescribed dose of radiation to a target region of a patient, for example, to a tumor or other cancerous tissue. The target region may be imaged prior to the administration of radiotherapy, and a treatment plan may be formulated based on, e.g., the size, location, and/or orientation of the target and the surrounding structures, among other things. A linear accelerator (linac) or other suitable radiation delivery device may then be used to deliver radiation to the target region of the patient. The linac may direct photons (e.g., X-rays), electrons, or other subatomic particles toward a target, such as a tumor.
After initial images of the target are acquired, the location and/or orientation of the target region may change. For example, the patient may shift during transfer to the treatment room, during movement within the treatment room (e.g., positioning on a couch, bed, or table), or during the administration of radiotherapy. For example, a patient may have voluntarily or involuntarily movements due to regular biological processes, including, e.g., breathing, swallowing, blinking, twitching, peristalsis, digestion, beating of the heart, coughing, passing gas, or other movements.
Additionally, tracking anatomy across different radiation therapy treatment sessions (called fractions) may be complicated, because a patient may lose or gain weight between each fraction, a target region (e.g., tumor) may change size (e.g., shrink or get larger), or the anatomy around the target region may affect the position of the target region (e.g., the volume of a patient's bladder may change across fractions, affecting the location of surrounding structures).
Changes in the location and/or orientation of the target region may reduce the efficacy of radiotherapy. For example, if the actual orientation or location of the target region is different than the assumed orientation or location based on prior imaging and/or inaccurate alignment with prior imaging, then the correct dose of radiation may not be delivered to the intended target region. Additionally, surrounding healthy structures may receive radiation instead of, or in addition to, the intended target region. Exposing the wrong area to radiation may ultimately harm or kill surrounding healthy cells. Further, it may be desirable to match images of the patient's anatomical structures taken across fractions and/or to an original CT image taken of the patient to track the location of dose delivery and the overall dose delivered to the patient.
Accurate image alignment and tracking techniques may be desirable for radiotherapy to account for movement of anatomy (e.g., movement of a tumor or movement of surrounding healthy structures) and changes in anatomy (e.g., changes in tumor size or weight gains or losses) over the course of treatment (e.g., planning, prepping across fractions) and as radiation is delivered to the patient during a fraction. It may also be desirable to track the location of dose delivery and the overall dose delivered. Accordingly, a need exists for systems and methods that improve the ability to align and compare images of a patient taken at different points in time.